GRINDING WHEELS 
435 
cone pulleys or. better yet, to employ a shunt-wound electric 
motor and rheostat and thus obtain a variation in speeds. 
One of the greatest troubles we encounter when dealing with 
abrasive wheels or papers or powders is the non-uniformity of 
grain size. A few large grains present, often a single one in a 
small area of the grinding surface, will so deeply scratch the 
specimen as to render its proper preparation almost impossible. 
If a wheel is found upon trial to have any such projecting par¬ 
ticle the wheel should be abandoned at once, and never be 
employed save for the crudest sort of grinding. It is this dif¬ 
ficulty which leads many workers to discard abrasive wheels for 
all save the roughest dressing of a specimen and use only laps 
fed with very carefully ground, sifted and floated abrasive 
powders. 
Laps may be either horizontally or vertically driven. The 
beginner will find that satisfactory surfaces are obtained easier 
upon the horizontal lap, but it is open to the objection that it 
does not readily clear itself and any dust or dirt falling upon 
it or any large particle of abrasive will be apt to deeply groove 
the specimen. The vertical lap on the other hand is difficult to 
keep charged with pasty abrasive or thin suspensions of abrasive 
and polishing powders. 
In the case of soft alloys, facing to a smooth surface is most 
easily accomplished by means of files, rough dressing with a 
lo or 12-inch bastard cut file and passing to an 8 or lo-inch 
single cut. With moderately soft materials such as brass, lay¬ 
ing a single cut mill file flat upon the work bench and pushing 
the specimen down the file against the cutting edges will be 
found to yield good smooth surfaces with less practice and skill 
than by holding the specimen in a vise and pushing the file. 
The specimen should be pushed lengthwise of the file with gentle 
pressure until it reaches the tang end, then lifted off; the file 
turned edgewise and struck a sharp blow upon the bench to 
remove filings, again laid flat and the specimen again laid upon 
the file and gently pushed toward the tang end, and the process 
repeated until a small plane surface is obtained. Specimens 
should never be rubbed back and forth upon an abrasive surface. 
